10 



REPORT — 1839. 



When a slip of sensitive paper is exposed to a highly concentrated 

 spectrum, a picture of it is rapidly impressed on the paper, not merely 

 in black, but in colours, a fact which I mentioned nearly two months 

 ago, and which observation of mine seems to have been alluded to 

 (though in terms somewhat equivocal,) by M. Arago in his account 

 of Daguerre's process. In order to understand what follows, it will be 

 necessary to describe the colours so depicted. The red is tolerably 

 vivid, but is rather of a brick-colour than a pure prismatic red. And 

 what is remarkable, its termination falls materially short of the visible 

 termination of the spectrum. The green is of a sombre, metallic hue, 

 the blue still more so, and rapidly passing into blackness. The yellow is 

 deficient. The whole length of the chemical spectrum is not far short 

 of double that of the luminous one, and at its more refrangible end a 

 slight ruddy or pinkish hue begins to appear. The place of the extreme 

 red, however, is marked by no colour, thus justifying, so far, the ex- 

 pression which M. Arago is reported to have used in speaking of my 

 experiments — " Le rayon rouge est seul sans action." 



It is impossible in this climate to form a brilliant and condensed 

 spectrum without a good deal of dispersed light in its confines ; and 

 this light, if the exposure of the paper be prolonged, acts, of course, 

 upon every part of its surface. The coloured picture is formed, there- 

 fore, on a ground not purely white, but rendered dusky over its whole 

 extent, with one remarkable exception, — viz. in that spot where the ex- 

 treme red rays fall, the whiteness of which is preserved, and becomes 

 gradually more and more strikingly apparent, the longer the exposure 

 and the greater the consequent general darkening of the paper. 



In the figure, R V is the luminous, and a d the chemical spectrum. 

 Of this the portion a y is white, its middle corresponding to the extreme 

 red of the luminous spectrum ; y ^ is red ; ^ e green, passing rapidly 

 through a shade of extremely sombre blue e ^ into black, which occu- 

 pies the whole space from ^ to rj. 



The above is not the only singular property possessed by the ex- 

 treme red rays. Their action on paper already discoloured by the 

 other rays is still more curious and extraordinary. When the spectrum 

 is received on paper already discoloured slightly by the violet and blue 

 rays only, they produce, not a white, but a red impression, which, how- 

 ever, I am disposed to regard as only the commencement of a process 



